Civil Engineering - Water Supply Engineering - Discussion

Sedimentation tanks are designed to remove a part of organic matter from the sewage effluent coming out from the grit removal chambers or detritus chambers. A plain sedimentation tank under normal conditions may remove about 60-65% of the suspended solids, and satisfy 30-35% of the BOD from the sewage.

There are two types of sedimentation tanks: Intermittent settling tanks: It also called quiescent type tanks and these are simple settling tanks which store sewage for a certain period and keep it in the complete test. The depth of the intermittent tank is 2.5 to 6 m.

Continuous flow type tank: The supply in such tanks is consistent. In these tanks, water enters from one end and comes out from the other end. The depth of the continuous tank is 3 to 4.5 m.

Hence, the maximum depth of the sedimentation tank is 6 meters.

Detention period: for plain sedimentation: 3 to 4 h, and for coagulated sedimentation: 2 to 2.5 h.

Velocity of flow: Not greater than 30 cm/min (horizontal flow).

Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m (common) maximum 100 m. Breadth= 6 m to 10 m.

Circular: Diameter not greater than 60 m. generally 20 to 40 m.

Depth : 2.5 to 5.0 m (3 m).

Surface Overflow Rate: For plain sedimentation 12000 to 18000 L/d/m2 tank area; for thoroughly flocculated water 24000 to 30000 L/d/m2 tank area.

Slopes: Rectangular 1% towards an inlet and circular 8%.

For continuous flow type tank -- Depth 3 to 4.5 m,
For intermittent settling tank -- Depth 2.5 to 6 m,
Therefore, max = 6m.

And Settling velocity are independent of the depth of tank but as per nptel notes depth can be between 2.5 to 5m.

1.8 m is minimum and 6m is maximum.

Ref: According to water supply engineering by S K Garg.

Sedimentation tanks are independent of the depth of the tank. Max depth may be anything.

Maximum depth kept- 3m.

Maximum depth is kept limited-6m.

The depth of tank should be 3 to 4.5 meters.

However, the practically max value is 3 m.

According to IS code 2m is minimum depth.